If individual message is enabled then tab will be displayed on Order Edit Page and Customer Edit Page as shown in below image.
Below is image of individual message form for customer from administrator panel in nopCommerce. You can send individual message to customers from two places. First one is Customers → Customers → Edit (select any customer for edit) → Send message (tab). Second one is Sales → Orders → View (select any order to view) → Send message (tab). If private message is enabled, then you can also send same message as private message to customer.